Digital transmission is becoming important in the processing of speech for telephone communication systems including digital radiotelephone systems. Analog speech signals are converted into digital data for transmission and reconstructed into analog speech at the termination end of the transmission. Digital speech processing has many offsetting requirements in which speech quality must be offset against available memory, processing speed and bandwidth requirements.
Because of growing demand and the conversion to digital transmission, the required bandwidth for a telephone transmission channel has increased significantly. This has lead to the development of digital data processing techniques to reduce the bandwidth required for a particular level of subscriber capacity for a telephone transmission channel.
Increasing the number of channels for a particular bandwidth has been achieved by application of techniques to reduce the required bit rate to accommodate a given capacity. These techniques include many data compression schemes for reducing the number of bits required to specify a particular analog signal. For example straight forward conversion of an analog speech signal to digital from would require a very large bandwidth for transmission probably making such transmission uneconomical. By compressing the digital data, the required transmission bandwidth can be significantly reduced.
Many various coding techniques have been devised to achieve the desired data compression to reduce the required bandwidth for transmission. Many of these techniques require the conversion of a digitally encoded signal to some preset quantized signal. Such a signal is derived by comparing an initially digitized signal with a limited set of preselected digitally encoded levels contained in a memory designated codebook. The initially digitized signal is compared with each codebook entry and the codebook entry best representing the initially digitized signal is selected for transmission or for further processing prior to a subsequent transmission.
A search for a matching stored code using an element by element comparison is time consuming if the codebook is large and is hence unsuitable for speech transmission systems that operate in real time. The use of small easily search codebooks are unacceptable because it fails to provide enough information to accurately reproduce the original analog signal. Many alternative codebook search techniques have been developed to speed up the process and permit searches in the real operating time of the speech processing system. Many of these techniques take advantage of known properties of speech signals and known properties of digital codes. These techniques have included the prior techniques of carefully preselecting the codebook entries in light of the expected characteristics of the speech signal to enhance the subsequent searching processes and various distortion measuring techniques for measuring expected distortions between the actual and coded signal.